Effects of flux additions on inclusion removal and microstructure in electron beam button melting of Udimet 720

Abstract

An investigation has been made of the effects of additions of flux on the removal of non-metallic inclusions and on the macrostructure and microstructure developed during the electron beam button melting of alloy Udimet 720. The fluxes used were based on the alumina–lime–silica system and were added either prior to the start of the drip melting procedure or after drip melting into the copper crucible had commenced. Flux additions led to a decrease in the sulphur content of the alloy and to a decrease in inclusion content in the bulk of the button. The distribution of inclusions that collected on the upper surface of the buttons was dramatically modified by flux additions which distributed them to the perimeter of the top surface rather than collecting them into a central ‘raft’ as occurs in the absence of the flux. The flux additions also led to a macroscopic change in the shape of the button, giving a concave top surface profile instead of the normal convex form and to a smooth, rather than rough, surface at the button/mould interface. The presence of a molten flux layer between the button and the crucible wall lowered the the heat transfer coefficient giving rise to dendritic growth instead of the fine equiaxed chill zone found in buttons melted without flux.